1. Field of the Invention
The present invention relates to a packet data transmission in a Wireless Local Loop (WLL) system and, more particularly, to a method for preventing a call collision of packet data.
2. Background of the Related Art
Generally, a Wireless Local Loop (WLL) System provides respective subscribers with wireless subscriber lines, unlike the background art wired subscriber network that connects an exchanger and the respective subscribers using wired subscriber lines. According to the background art, the voice band data transmission speed is limited to 64 kbps. However, a digital wireless line subscriber network can provide high speed digital data service, more than 64 kbps without using a modem. Also, a service supplier can reduce the installation expenses, installation terms, and various additional services using the WLL system. For example, the WLL system can provide Plain Old Telephone Service (POTS), data service, and Integrated Services Digital Network BRD (ISDN BRD) service.
The WLL system can be technically divided into a WLL exclusive technique, a cellular technique, and a cordless technique. The cellular technique is considered the most suitable idea for embodying the WLL system. Also, the WLL system has similar characteristics to those of a mobile communication network regarding the use of a wireless channel as a communication media. However, the WLL system has better a propagation environment than that of the mobile communication network because the WLL system does not have roving characteristics. In the WLL system, an antenna of a terminal can be installed at a high position, such as a rooftop. Therefore a line-of-sight is ensured and the signal propagation suffers a small loss of about 20 dB/decade. Therefore, a broad region can be served with the same transmission power as that of the mobile communication network.
Fading phenomena caused by multiple channels happens even less than in the mobile communication network, of a point-to-mobile station method, because the WLL system uses a point-to-point communication method. Also a hand-off is not generated because the WLL system is a fixed wireless communication network and the call disconnect is not generated. Therefore, an additional wireless channel for hand-off is not necessary.
A radio port and a terminal in the WLL system exchange data by establishing a communication path. The exchange method can be divided into a circuit exchange method, which sets communication paths whenever data is transmitted between transmission and receive terminals; a message exchange method, that is, an accumulation exchange method, in which an exchanger receives a message from a caller and transmits the message to a receiver; and a packet exchange method that transmits information as a packet unit, which is made by dividing data into packets and adding a header to the respective packets. With the packet exchange method, the exchanger selects an appropriate path according to an address of the receiver and transmits the packet. The packet exchange method includes a datagram method in which the respective packets are transmitted independently, regardless of their order. The datagram method has advantages such that a call setting step can be omitted when the path is set and it is flexible with respect to the call collision of the packet data.
The basic structure of the packet data transmission system, including the background art WLL system, will be described with reference to FIG. 1. The data transmission system including the background art WLL system comprises terminals A–C (100–120), which generate packet data; a WLL system 200 that transmits the generated packet data; an internet network 300 that transmits the packet data received from the WLL system 200 to an object terminal (not shown); a Public Switched Telephone Network (PSTN) network 400 that receives the packet data, transmitted from the WLL system 200, and transmits it to the object terminal (not shown); and an Operation Maintenance Center (OMC) 500 that serves the packet data transmitted from the WLL system 200. The WLL system 200 is located at a final end among the network elements and comprises a Radio Interface Unit (RIU) 210 that performs a Radio Frequency (RF) sending/receiving function for sending/receiving information through a radio interface, transmits a signal that is changed from RF to baseband to a subscriber interface, and converts a baseband signal to an RF signal; a Radio Port (RP) 220 and 221 for setting a call, restoring a call, and processing data related to power control; a Radio Port Controller (RPC) 230 for managing the RP 220 and 221 and controlling a message process needed to originate/terminate the call process; and a Radio Port Operation & Maintenance Center (RPOM) 240 for managing the RPC 230, the RP 220, 221, and the RIU 210 and performing subscriber and service management. In addition, the RP 220 includes a Packet Data Management Unit (PDMU) 220A that participates in the call connect between the terminal and the RP.
The packet data transmission method will be described with reference to FIGS. 1 through 4 as follows. The RP 220 transmits preamble data, used for synchronization, to terminals A–C 100–120 through a pilot channel and RIU 210 (S100). The RP 220 can transmit the preamble data to the terminals A–C 100–120 through the RIU 210 and the pilot channel because a PN code and a Hadamard code of the PDMU 220A and of the terminals A–C 100–120 are same.
The RP 220, which transmitted the preamble data to the terminals A–C 100–120, checks whether or not it is synchronized (S110 and S120). If so, the RP 220 transmits controlling information, carrying a terminal ID, to the terminals A–C 100–120 through a signal channel (S130). However, if the RP 220 is not synchronized, the RP 220 transmits preamble data to the terminals A–C 100–120, again, for synchronization.
The terminals A–C 100–120 receiving the controlling information identify whether there is an error in the controlling information (S140 and S150). If there is no error in the controlling information, the terminals A–C 100–120 check whether the terminal ID in the controlling information corresponds to their own ID (S160 and S170). If the IDs correspond with each other, terminals A–C 100–120 transmit packet data to the RP 220 through the RIU 210 and the signal channel (S180).
The structure of the general packet data will be described with reference to FIG. 2 as follows. The packet data comprises a protocol head 10, which is the controlling information part, and data 20. The protocol head 10 comprises IP addresses of the originating/terminating sides, identifying an origin and a destination, and Media Access Control (MAC) addresses (not shown), which are checksums for detecting data error, that is, physical addresses.
However, if there is an error in the controlling information or the terminal IDs do not correspond, terminals A–C 100–120 throw away the received controlling information and perform the synchronization operation again (S220).
The RP 220 transmits the packet data received from terminals A–C 100–120 to the RPC 230 (S190). The RPC 230 transmits the packet data to the object terminal (not shown) through the PSTN network 400, if the packet data transmitted from the RP 220 is voice packet data, and the object terminal (not shown) serves the transmitted voice packet data. However, if the RPC 230 receives general packet data, the RPC 230 transmits the general packet data to the object terminal through the internet network 300 and the object terminal serves the transmitted general packet data. Also, if the RPC 230 receives Operation & Maintenance (OM) data, the RPC 230 transmits the O&M data to the RPOM 240. The RPOM 240 transmits the O&M data to the OMC 500, and the OMC 500 serves the transmitted O&M data (S200 and S210).
However, when the packet data is transmitted using the packet exchange method, in the transmission system including the WLL system, the terminals A–C 100–120 share the same signal channel, unlike in the circuit exchange method. Therefore, as shown in FIG. 3, it is possible for all of the terminals A–C 100–120 to attempt a call connection at the same time. In this case, a call collision is generated between the terminals A–C 100–120 and the RP 220. Therefore, the packet data transmitted from terminals A–C 100–120 to the RP 220 may be lost or distorted.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.